Controlled selective gene expression plays the key role in embryonic development and cellular differentiation. We plan to study the control of gene expression in embryonal carcinoma cells (ECC) induced to differentiate into entodermal cells either spontaneously or by retinoic acid. Extensive homologies between normal embryonic development and in vitro differentiation of ECC warrant the use of the latter as the valid and more accessible experimental model. To analyze the existence and the extent of post-transcriptional control of gene expression we will compare the sequence complexities of hnRNA and mRNA from both ECC and entodermal cells by molecular hybridization. Several genes differentially expressed in ECC and their differentiated deviations have been cloned. The level of cytoplasmic mRNAs detected by these clones is substantially reduced in differentiated cells while the level of primary nuclear transcript is the same in both cell types. It was determined that the stability of the investigated mature mRNAs is decreased in differentiated cells, thus providing evidence that mRNA stability is involved in control of gene expression during differentiation.